52 GENERAL ACTIONS OF DRroS 



combined. When spasm of the bronchioles exists, as in 

 asthma, and sometimes in bronchitis, the nitrites are partic- 

 ularly valuable. 



Drugs Acting on the Urinary Organs. 



Diuretics are drugs increasing the secretion of urine. 

 Three factors are concerned with the urinary flow. (1) The 

 composition of the blood. (2) The state of local (renal) 

 and general blood pressure. (3) The activity of the 

 renal cells. The renal mechanism influenced by drugs com- 

 prises : 



(1) The mcdpighian glomerideSy which excrete water, salts 

 and some excrementitious matters from the blood by 

 osmosis. Their activity depends upon their blood supply, 

 which is increased by agents causing dilatation of their 

 afferent vessels, or contraction of the efferent vessels ; and 

 by drugs increasing general blood tension. Drugs aug- 

 menting the mass of blood and tension in the malpighian 

 bodies enlarge the surface of cubical epithelium covering 

 the capillary loops and promote osmosis of fluid into the 

 cavity of the malpighian capsule. 



(2) The nucleated polyliedrol cells lining the convoluted tubes, 

 —These secrete the solid products resulting from the retro- 

 grade metamorphosis of nitrogenous bodies circulating in 

 the blood, as urea, together with water. 



(3) The constricted tubes. — These regulate the urinary 

 secretion by either impeding its passage by constriction of 

 their walls, thus aiding absorption, or by their active 

 peristalsis facilitating the flow of urine. 



(4) Nervous mechanism, — This governs the secretory 

 activity of the cells of the convoluted tubes, regulates the 

 tension in the vessels of the malpighian bodies, and possibly 

 controls the unstriped muscle of the constricted tubes. It 

 comprises : 



